This invention relates to a large aperture cylindrical array of sonar transducers wherein the transducers comprise polymer hydrophones. One prior-art technique for construction of such arrays, shown in FIG. 1, is to employ the cylindrically curved array segments 20 of FIG. 2, each segment typically containing two rows each having fourteen polymer hydrophones 21. Each polymer hydrophone 21 is typically five inches square with a half-inch spacing between adjacent hydrophones of a segment. Sixteen circular-arc segments are arranged in side-by-side configuration to form the cylindrical array 10 of FIG. 1 with the spacing between adjacent hydrophones of adjacent segments also being spaced by one-half inch. The illustrative distance of one-half inch is thought to be the minimum practical design geometry. Edge-to-edge distance between hydrophones of adjacent segments should be the same as edge-to-edge distance between the hydrophones of the same segment in order to maintain symmetry. The preceding arrangement results in a cylindrical array whose length in the direction of the axis of the cylinder is substantially twice that of the arc of the array. The acoustic properties of the array depends upon minimum symmetrical spacing of the hydrophones in both their circumferential and longitudinal spacing. The array segments are constructed with two rows of hydrophones so that the space between the adjacent corners of four adjacent hydrophones in an array segment may be utilized for fasteners and electrical connectors without disturbing this symmetry. Acoustical array segments 20 when assembled into an array 10 must have a smooth faired outer surface between segments and at the edges of the array to minimize water flow noise which degrades acoustic performance.
Prior-art designs of array segments 20 are characterized by each segment being formed with a cylindrical curve in its long dimension. The array segments are assembled in side-by-side relationship on a cylindrical signal conditioning plate to form an array 10. Signal conducting wires are connected to each transducer element and are assembled into a cable (not shown) which exits the back of each segment. This prior-art design has the following disadvantages. The gap between each array segment must be filled void-free and precisely faired to minimize flow noise. This is a difficult and expensive task. The gap filler is usually of a material which is less than perfectly homogeneous with the outer rubber/air decoupler of each array segment which results in acoustical degradation due to scattering. Also, curved array segments are difficult to manufacture and their assembly is difficult and expensive. For example, the tooling for a curved surface is expensive. Also, the array segment 20 assembly requires the fasteners and locating pins on the cylindrical signal conditioning plate (to which the segments 20 are attached) to be parallel to each other (and thus not radial) requiring expensive spot facing of components. Also, manufacturing and assembly operations which require indexing are difficult. Also, operations such as pouring encapsulants to fill counterbores and the gap between array segments are more difficult and time consuming on a curved assembly of segments.
Hitherto, the curved array segment 20 design has been thought to be necessary to maintain symmetry of the hydrophones in that with the curved design, each hydrophone 21 is normal to the radius 22 of the cylindrical array and equidistant from the center line 23. These latter features were considered necessary to allow versatility in combining appropriately delayed outputs of each of the transducer elements 21 in order to generate a multiplicity of beams from one cylindrical array 10.